Electron reflection seam tracker



5 o 0, 'l 2 .u R

R E K LC Am UT M A mw. mw FI am wm C N Em. E L E Dec. l 5., 1970 Oriftnal F'i Tod Deo.

3 Shams-Sheet 1 FIGJ f /f//V/f/r 8 O l 2 Z FIG.2 b

FlG.3o

levi. l Y1 I f 'I ,4- bo SCANNING INVENTORS RUCHARD H. GRAHAM EDWARD C. VIINGFIELD ATTORNEYS ba i TIME

DCCJS, 1970 E- C. W|NGF|ELD ETAl. Re. 21,005

ELECTRON REFLECTION SEAM TRACKER Original Filed Dec. 9, 1965 5 Sheets-Sheet 2 W f* @4f FIGA ,a C: :l /56 -VJFL/ 28 E E 72 l /62 A /70 U -I N gli I ,o j u [V96 F|s.5 98 I: sus /2-A ,6 I 92 ,a coMPAmsoN A 344.7/ Y Kad 94 ao F |G.7 ,04

\\\\\ xxi INVENTORS RICHARD H. GRAHAM EDWARD C. IIHGFIELD ATTORNEYS DCCJS., 1970 E, Q W|NGF|ELD ETAl. Re. 21,005

ELECTRON REFLECTION SRAM TRACKER Original Filed Dec. 9. 1965 3 Sheets-Sheet s I'IG.B

FIG. IO

10 AMPLIFIER EDWARD C. VIINGFIELD ATTORNEYS United States Patent O Int. Cl. H05b 7/18 U.S. Cl. 219-121 16 Claims Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

ABSTRACT 0F THE DISCLOSURE A device is described for tracking and welding a seam formed between abutting workpieces to be welded along the seam by a beam of electrons wherein a detector is used which provides a voltage in response to reflected electrons generated by a beam impinging upon the workpieces in the vicinity of the seam. The beam crosssectional size is commensurate with the separation of the workpieces along the seam for at least partial entry of the seam by the beam. The beam is deflected over the surface of the workpieces across the seam to generate a detectable pulse from the detector corresponding to the entry of the seam by the portion of the beam and the thus amplitude modulated detector voltage is combined with a signal representing the deflection voltage to produce a signal representative of the deviation in time between the pulse in the detector voltage and a recurring value in the deliection voltage corresponding to a desired position of the seam. Thereupon the beam and the seam are moved relative to one another either by movement of the workpieces or by dellection of the seam to bring the beam directly onto this seam.

This invention relates generally to electron beam equipment and, more particularly, relates to an improved arrangement for the detection of bulk discontinuities in the workpieces of such equipment and the utilization of bulk discontinuities for the generation of information signals useful in control of such electron beam apparatus.

In electron beam equipment, electrons accelerated through high potentials impinge upon the workpiece in order to perform various working functions thereon such as, for example, welding of abutting workpieces along a seam. In such equipment, it is usual to provide a controllably movable workpiece stage to permit adjustment of the workpieces with respect to the gun. In addition, the position of beam impingement is usually controllable (though the positional adjustment is of smaller amplitude) by deflection coils or plates.

It is necessary, however, to have certain information concerning the impingement position in order to properly control the electron beam processing. For example, when welding along a seam, it is necessary to provide information so as to permit control over the deliection of the electron beam to ensure coincidence of the seam and the impingement position of the beam.

Optical observation of the impingement position has been used. However, with optical observation of the impingement position, certain diiculties have been encountered. Among these is the difficulty of maintaining clarity of the optical system over periods of use. In high energy beam apparatus, volatilized material from the workpiece deposits on the optical components, obscuring Cit Re. 27,005 Reissued Dec. 15, 1970 Nice the image after relatively short periods of operation. Other types of seam trackers have been proposed. However, these usually require ancillary processing, such as painting precise reliective guide lines along the seam, which introduces added processing costs and reduces equipment capability.

In electron beam apparatus, the electron back-scatter has been utilized to provide information signals. Electron back-scatter generally is composed of two elements. The first contribution to the total effect of back-scatter is provided by a secondary emission which is a low energy electron emission (below 50 ev.). The remaining contribution is provided by higher energy secondary emissions which are called reliected electrons. For the purpose of this specification, electrons having an energy below 50 ev. will be called secondary electrons and those with higher energies will be termed reected electrons although it must be recognized that in practice, the boundary cannot be defined quite this precisely.

For example, in connection with the surface studies of opaque conductors by electron miscroscopes, use has been made of the election back-scatter from the primary electron beam impinging on the surface and displaying the back-scatter as visual display for observation. Contrast of the visual display indicative of variation in specimen surface conditions is attained by utilizing differential backscatter from different points on the surface during scanning thereof.

In electron beam microscopy, secondary emissions are depended upon in order to obtain the desired surface contrast because electron secondary emission shows a dependence upon primary beam-to-surface angle which is used to enhance the contrast in the scanning microscope.

The direct application of this technique to control electron beam apparatus for working materials has been attempted. However, such applications have not been as successful as desired. For example, a surface scratch on the work specimen will often provide the same contrast as the seam. Thus, control of the impingement position of a welding beam in response thereto is ambiguous and control fails.

However, when the back-scatter is composed primarily of rellected electrons, it has been found that the high speed electron emission current is relatively independent of the angle between the reflecting surface and the primary beam. Further, it has been found that when a beam is scanned over the surface of a workpiece, bulk discontinuities (such as a seam or hole having a high depth-towidth ratio) can be detected from the back-scatter of the reected electrons with rather high suppression of the detected signal variation due to surface variations. It should also be noted that it has proven experimentally possible to detect the seam signals even when there is no lineI of sight path between the seam and the detector. This is due to multiple scattering of electrons from the chamber walls.

It is, therefore, a primary object of the present invention to provide an improved apparatus for the detection of bulk discontinuities in a workpiece by means of an electron beam and to generate a control signal in response thereto.

It is a further object of this invention to provide an improved arrangement for the generation of a control signal responsive to bulk discontinuities and control means responsive to such signal to control the impingement position of such beam.

In addition to detection of seams, the signal generated by bulk discontinuities `may be used for the purpose of generating information and control signals. For example, by using calibrated discontinuities, information concerning the diameter of the beam can be obtained. This information signal can be utilized to control the focusing coils of the electron beam apparatus. Similarly, information concerning the outline irregularity of the beam can be obtained from calibrated bulk discontinuities.

It is, therefore, another object of the present invention to provide an improved method and apparatus for the generation of information signals from bulk discontinuities in a workpiece in order to generate control signals for control of electron beam apparatus.

In accordance with these objects, there is provided, in a preferred embodiment of this invention, a scanning beam comprising a focused, collimated beam of electrons. Ihe electron beam is of reasonably high speed (falling through a typical acceleration potential of 100 kev.) but is of an intensity below that utilized for working of the material. Means are provided to sweep the electron beam across the surface of interest on the workpiece. Detector means are provided to collect the reflected electrons backscattered from the surface of the workpiece. As the sweeping beam encounters a bulk discontinuity, the backscattered electron current will drop, due to the Faraday cup effect of a bulk discontinuity on bacltscatter` This information signal may then be utilized for beam deflection or beam focusing techniques.

For example, the information signal may be conveniently coupled as the. error signal in a servo loop to align the seam with a second electron beam of welding intensity so that the welding beam precisely and accurately tracks the seam during welding without the necessity of optical observation of the workpiece. Al-

ternatively, a bulk discontinuity of known dimension may be utilized to give information about the beam diameter, in which case the information signal may be utilized to control current in the focusing coil. This technique may be used in a more sophisticated manner by rotation .t

of an extending bulk discontinuity about a point coincident with the impingement position of the beam. The information signal generated will, therefore, provide information related to the angular displacement of the calibrated bulk discontinuity, thereby to provide information concerning the symmetry of the impinging electron beam about the center of rotation.

Preferably, the detector is constituted as a hemispherical or other body subtending 211 radians centered on the position of impingernent of the scanning beam, thereby to suppress amplitude variation in the backscatter due to changes of angle of incidence of the beam and increasing the information dependent upon the existence of bulk discontinuities. In some applications, however, such as out-of-vacuum working of materials, a planar detector may be utilized to simulate hemispherical coverage and provide an acceptable information signal.

Having briefly described this invention, it will be described in greater detail along with other objects and advantages in the following portions of the specification, which may best be understood by reference to the accompanying drawings, of which:

FIG. l is a cross sectioned elevation view of an apparatus in accordance with the present invention;

FIG. 2a is a partially sectioned view to enlarged scale of the detector in FIG. l showing the backscatter pattern and FIG. 2b is a diagrammatic view of the detector illustrating the factors involved in the derivation of Equation II.

FIG. 3a is a cross sectioned view to enlarged scale of a bulk discontinuity useful in explanation of the detector signal variation in the discontinuity and FIG. 3b is a plot of detector response at the bulk discontinuily;

FIG. 4 is a schematic diagram showing a typical system for position control using the apparatus of FIG. l;

FIG. 5 is a schematic diagram of a typical system for beam control using the apparatus 0f FIG. 1;

FIG. 6 is a broken away elevation view of another 4 embodiment of the detector useful in the practice of the present invention;

FIG. 7 is a cross sectioned view partially in schematic form of the detector shown in FIG. 6;

FIG. 8 is a plan view of a planar detector useful in the practice of the present invention;

FIG. 9 is a cross sectioned View along lines 9-9 of FIG. 8;

FIG. 10 is a plan view of a second planar detector; and

FIG. 1l is a cross section taken on lines 11-11 of FIG. l0.

In FIG. l, there is shown a typical electron beam Welder having an electron gun consisting of a filament cathode 1l), a grid electrode 12 and an anode electrode 14. The filament is heated t0 generate a cloud of electrons which is accelerated by the cathode-anode potential. The grid 12 is biased to control the intensity of the generated electron beam. The electron beam 16 then passes through alignment coils 18 and 2() and through an aperture plate 22 which limits the beam path to the aperture. A magnetic lens 24 is so positioned and the field intensity thereof is so controlled as to focus the beam 16 upon the workpiece 26. The position of impingement can be controlled by the deflection coils 28. The workpiece is mounted on a table 30 which can be moved in the plane of the paper and in a plane perpendicular to the paper by conventional tracking means not illustrated in detail.

As the impinging beam strikes the workpiece, electrons 32 will be backscattered from the workpiece, which electrons are collected on the hemispherical detector 34. A shield 36 may be provided on the outer surface thereof to shield the detector from stray electrons as, for example, backscattered from an adjacent welding beam. Leads 38 are provided to bring the derived signal on the detector to the outside of the welding equipment through the wall of the vacuum chamber.

The 2r detector is shown in greater detail in FIG. 2a and consists of a hemispherically shaped detector surface 34. The surface is developed as a body of rotation of radius R swung from a center corresponding to the central position of the impinging beam as it passes through the aperture 40. The aperture 40 must be sufficiently wide to permit the desired scanning by the electron beam 16, illustrated in its central position and extremes of deflection during scanning.

It has been found that the secondary emission consists to a substantial degree of electrons of the reflected type. These high speed reflected electrons travel in the straight lines from the surface of the workpiece to the detector as illustrated by the arrows 27 and the angular dependence amplitude of reflected electron current as a function of the angle between the surface and the current density vector is very closely approximated by Equation I:

j=jr sin 0 where:

jr=max. current density vector at radius r=Cr2/r2 jzreected current density vector (izangle between the surface and the reflected current density vector C=a constant for a given beam current and target `material and r0=radius of beam spot on the target surface If the detector is a hemisphere of radius r, the element of current at the detector surface is dizjdA. AS` sume r r0 where dA=element of hemisphere surface (see FIG. 2b) Area=r2 cos ddq and j=jI sin 6 from Equation I then dizjrr2 sin 0 cos 0d6d where 6 and n are the radius vector coordinate angles in spherical coordinates i=1rjr2[l sin2 00] Equation II: i=1rjrr2[cos2 00] where =the obstruction angle formed if the detector is not a full hemisphere.

Currents intercepted by non-hemispherical coaxial detectors may be determined by projecting their areas into an equivalent hemisphere and solving Equation II. A cylindrical detector is an example of this type. By generating a surface fo-rmed by rotating the radius vector r (from the center of any hemisphere to the outer edge of the nonhemispherical detector), that portion of any hemisphere surface enclosed by the intersection of the two surfaces is the equivalent hemisphere of the non-hemispherical detector and Equation II applies for circularly symmetrical coaxial planar detectors. Other shapes or orientations must be analyzed individually.

The intensity distribution given by Equation I holds for reflected electron backscatter and is substantially independent of changes in the angle of incidence between the beam and the surface. This is quite unlikey the case of secondary electron emission which may show a high dependence on beam-surface angle and is used in scanning microscopes for the enhancement of contrast of surface variations.

As can best be shown in FIGS. 3a and b, the scanning, beam 16 is swept across a bulk discontinuity 42 of the workpiece 26, as indicated by arrow 44. A bulk discontinuity may, for example, consist of the space between abutted workpieces along which the weld seam is to run.

As the scanning electron beam 16 impinges upon the hulk discontinuity, the current intensity of the reliected electrons will drop from the intensity at the workpiece surface to a minimal value as represented by the plot 46 in which reflected current intensity is plotted against time and in which the base line 48 corresponds to the reflected signal from the surface and the minimal signal is represented by the line 50. Under the conditions set forth in the drawing, the reflected electron intensity drops substantially to zero because of the Faraday cup effect of the bulk discontinuity. This effect is known and is explained by the absorption of substantially all of the rellected electrons within the walls of the bulk discontinuity. Thus, the backscattering from the surface drops to substantially reroI at the discontinuity, constituting 100% amplitude modulation of the backscatter.

It can be seen, therefore, that the beam will track only bulk discontinuities. Thus, surface scratches on the workpiece surface, which are almost inevitably present in practical Welding situations, will not cause the significant variation in the detector output signal caused by a bulk discontinuity.

The example given is the ideal situation in which the beam is substantially smaller than the dimensions of the bulk discontinuity. If the beam is wider than the bulk discontinuity as, for example, because the beam is used in out-of-vacuum welding applications, the signal variation will not be as great as that set forth in FIG. 3b. However, a signal due to backscatter amplitude modulation will still exist since a portion of the beam enters the bulk discontinuity and thus generates an information signal. The limits of detectability of the signal variation will depend in large measure on the application intended. For example, if the signal variation is lower than the noise level, detection can be made only by rather sophisticated comparison techniques. These may be useful in some applications Where the scanning information is processed upon tapes which then control the welding beam at a later time. If control is to be immediate, however, such techniques are often too time consuming to be practical.

The information signal may be utilized for positioning of the workpiece with respect to a welding beam adjacent to and aligned with the scanning beam by utilizing the circuitry of FIG. 4.

In FIG. 4, there is shown apparatus of FIG. l, including the detector 34 and the beam deiiection coils 28. A clock oscillator 52 is provided to serve as a source of reference signals. The clock signals are amplified by amplifier 54 and applied to one side of the coils 28. It should be noted that the y axis scan and the servo error signal may be applied to the same y axis deflection coil. The same clock output signal is provided to trigger a logic gate generator 56 to generate the sampling pulses 58 and 60 applied to the time selector 62. The selector is tripped by the sampling pulses to sample the scan signal from the detector 34 after amplification by amplifier 64. The clock is related to the undeflected beam position so that, if the seam is aligned with the undefiected position of the beam, the signal samples will be ndentical and no resultant difference in signal will be generated. The gated signals are then processed by the integrator 66 and amplified by amplifier 68 to develop an error signal on lead 70 which is amplified by amplifier 72 and applied to the deflection coils 28 in the opposite sense to the error signal. The same error signal is used to drive the servo motor 74 via lead 76 to position the workpiece so that the bulk discontinuity is aligned with the undeflected position of the incident beam. Thus, both mechanical movement of the stage and deflection of the beam are coincidentally employed for alignment. This permits rapid followup and wide scale deflection. The servo arrangement is considered conceptually conventional and other types of servo systems may be utilized.

Alternatively, bulk discontinuity detection may be utilized to develop information concerning the electron beam itself, which information may be provided in the form of the signal used to control the electron beam apparatus.

For example, the circuitry shown in FIG. 5 may be utilized for the development of information signals concerning the focus of the electron beam and used to control the focus conditions of the beam.

In FIG. 5, there is shown the electron beam apparatus consisting of a cathode 10 and an anode 12 to accelerate the electron beam 16 until it impinges upon a workpiece having a calibrated bulk discontinuity or slit 82 therein. In this sense, and as used generally throughout this specication, the term workpiece is applied to any object on which the beam impinges. such as a calibrated specimen used for information signal generation, and is not restricted solely to an object to be Welded. The bulk discontinuity may be a slit, the dimensions of which are accurately measured. A detector 34 is arranged above the impingement position and the detector signals are amplified by an amplifier 84 for application to a comparison circuit 86. A beam is swept across the calibrated slit by the deflection coils 28 energized by a sweep generator 88 running off a master clock 90. The clock signals are also applied to the comparison circuit over lead 92 so as to provide a timing base signal coincident with that used to control the sweep generator. The comparison circuit is provided to compare the width of `the detected signal with a signal whose width is predetermined from the beam velocity, the slit Width and the desired beam diameter under proper focus conditions. If the beam is properly focused, the detected signal width will coincide with the predetermined signal width. If, however, a difference exists, the difference can be used as an error signal which is amplified and rectified to direct current by the amplifier 94. A DC bias source 96 is serially coupled with the output from amplifier 94 by insertion in lead 98 to provide the steady state focusing current to the lens to which current is added or from which current is subtracted the error signal from amplifier 94 for continuous focusing control of the beam. Alternatively, if a minimum diameter beam is desired, the circuit may simply maximize the signal time derivative, to give the largest change in current per unit time.

Thus, with the circuit of FIG. 5, a calibrated bulk discontinuity is utilized to generate an information signal dependent upon the size of the beam spot as it impinges upon the workpiece. Comparison of the spot size with the predetermined spot size for proper focus can, thus, be utilized for generation of an error signal applied to the focusing coil for automatic focusing of the beam.

In general, this technique is most useful for developing information about the welding beam per se. Thus, the workpiece with the calibrated slit should be selected from material having a low Z (atomic number) and the high intensity welding beam should be scanned very rapidily. By positioning (either manually or automatically) the slit or iris at the edges of the beam current distribution, a continuous monitoring and/ or control of both beam focus and location may be achieved for Weld beams, without slit or iris destruction.

The workpice may be adjustably positioned within the electron beam apparatus so as to develop information concerning the beam characteristics at various points along the beam path if desired.

Similarly, although most useful with lower powered beams, the use of calibrated slits may be employed for studies of any electron beams. For example, less aberrations may be studied by providing a slit of the same dmisension as the desired beam diameter. The slit may then be rotated and the beam caused to impinge upon the slit at increments of rotation of the slit. The signals developed will, thus, be information signals related to deviations of the beam from its desired roundness at angular increments of rotation of the slit. This information may be used directly in the analysis of lens aberrations.

The circuitry of both FIGS. 4 and 5 may be utilized as calibration circuitry for operating electron beam equipment so that the operating equipment may be employed not only to determine the position of bulk discontinuities on the workpiece, but may also be used to measure the relative dimensions of such bulk discontinuities with respect to the calibrated dimensions.

As can be seen from Equations I and II, the ideal detector shape is a hemisphere with the impingement position coincident with the center of the hemisphere. In some applications, however, a detector which subtends a solid angle which is less than 21r radians may have certain practical advantages and may, under some conditions, be

capable of providing the desired detector output. For example, in FIGS. 6 and 7, there is shown a semi-circular detector which is useful in suppressing angular variations in one plane.

In FIGS. 6 and 7, there is shown the incident beam 16. The detector 100 is in the form of a semicircle of radius R, the center of which is coincident with the undeflected beam position. The surface of detector 100 is constructed as a Faraday cup in order to improve its efficiency in collecting all of the electrons which reach the surface from the backscatter. In this manner, the surface will not itself be a source of backscattered electrons which would increase the noise of the detector system and, thus, degrade performance. The detector 100 is preferably shielded by a grounded shield 102 to eliminate an output signal from random electron movement within the vacuum chamber, thus, to provide the detector with a field of View directed at the impingement position of the beam. The detector signal may be transmitted over lead 104 having coaxial grounded shielding 106 and developed across a load resistor 10S. A bias battery 110 may be provided so that the detector may be biased positively or negatively to improve its collection efficiency or to reduce its collection efhciency as dictated by the application intended.

It will be recognized by those skilled in the art that various detectors having a surface intermediate between a semicircle and a hemisphere may be utilized with varying degrees of efficiency in different applications when the mechanical shape of the detector must be changed in accordance with the requirements of the specific application.

In some applications, particularly out-of-vacuum work in which the electron beam exits from the vacuum of the generator and proceeds through ambient atmospheric pressure before impingement on the workpiece, the system requirements preclude using hemispherical or even semicircular detectors. In such applications, a planar detector is desired to decrease the space between the evacuated container and the workpiece and thus reduce beam spread by the atmosphere. In these applications, a planar detector may be constructed having a detector surface which will simulate the effect of a hemispherical surface and, thus, will serve to reduce the angular variation of the output signal thereby to make the output signal responsive to bulk discontinuities in a manner similar to a hemispherical detector. A planar detector of this nature is shown in FIGS. 8 and 9.

The detector consists of an insulated disc 116 symmetrical with the beam port 114, surrounded by a shield 118 and a Faraday cage shield 120 across the detector window. The detector surface may be covered by a low Z (atomic number) element such as carbon to reduce backscatter from the detector surface itself.

The detector shown has a radius equal to the distance from the workpiece and is, thus, equivalent to a hemispherical detector with a 45 obstruction angle by Equation ll.

Therefore, if the detector window is made in the form of a circle, the unit has a beam-surface angle compensated geometry. The design is useful in those cases where the hemisphere or semicircular detectors cannot be placed close to the workpiece. It should be noted that in the detector shown, the circular shape provides compensation for all types of surface roughness. In some cases, the surface scratches and machining marks are known to lie parallel to a single plane. In those cases, a simplified version of the planar detector may be utilized using only a flat strip detector placed perpendicular to the surface ridges and machining marks. This reduces the necessary size of the detector surface, but limits the application to that specified.

Under the precise conditions specified, the strip detector may be formed as a linearly developed Faraday cup type detector (e.g. the Faraday cup illustrated in FIG. 4 but extending linearly). Under such conditions, the spatial changes of lthe reflected electron vector array will not significantly influence the detector response.

Usually, however, it is not possible to satisfy the conditions precisely and the detector shown in FIGS. l0 and 1l may advantageously be employed.

In FIGS. 10 and 1l, there is shown a linear detector 121 extending linearly from a center beam port 122. The detector is developed as a Faraday cup having a U-shaped detector surface 124 enclosed within shield 126. A shield lead 128 is used to derive the signal from the detector surface. The shield is provided with a window 130 exposing the detector surface to the reflected electrons.

The window is shaped to provide compensating geometry to give an effective detector surface geometry so that the response therefrom is related to changes in the total electron backscatter and independent of changes in the spatial orientation of the backscatter pattern. For this purpose, the window width is varied in accordance with Equation III.

Equation III:

w= (Adly Sec 0=vy2|h2 Ad: where: w=width of window at distance y from origin (center of beam port) h=distance of detector from workpiece surface G=tan-1 E y wo=width at beam impingement Of course, a plurality of detector arms may be used for compensation in several planes.

This invention may be variously modied and embodied within the sco pe of the subjoined claims.

What is claimed is: 1. A device for tracking and welding a seam formed between abutting workpieces to be welded along the seam by a beam of electrons comprising means for generating and directing a focused beam of electrons at the seam, said beam having a crosssectional size in the vicinity of the seam commensurate with the separation of the workpieces along the seam for at least partial entry of the seam by the beam, means for generating a periodic signal for periodic deflection of said beam across the beam facing surface of the abutting workpieces and the seam, said signal having a periodically recurring value corresponding to a desired position of the seam relative to the beam, an electron back scatter detector positioned adjacent said workpiece surface and being shaped to receive omni-directionally moving reflected electrons generated by the beam impinging on the workpiece surface to produce a detector voltage having a magnitude representative of the received reflected electrons and varying with detectable pulsed magnitude in response to the entry of the seam by at least said portion of the beam as the beam crosses the seam,

means responsive to the detector voltage and the periodic signal for generating a seam position voltage representative of the deviation in time between the recurring value in the periodic signal and the pulsed part of the detector voltage, and

means responsive to the seam position voltage for moving the seam and the beam relative to one another in a direction tending to reduce said seam position voltage to a minimum and place the seam in the desired position.

2. An apparatus in accordance with claim 1 in which Said detector comprises a. conducting hemispherical surface having an aperture therein aligned with the beam for beam passage therethrough to impinge on said seam, with the concave portion of the detector facing the seam and the center of said aperture being substantially coincident with the impingement position of an undeflected beam and the desired position of the seam.

3. An apparatus in accordance with claim 1 in which said detector comprises a semicircular surface centered upon the impingement position of the undeflected beam, and in which said detector surface is developed as a Faraday cup.

4. An apparatus in accordance with claim 1 in which said detector comprises a linear detector surface, an electron shield surrounding said detector surface, said shield having a window cut in the surface thereof facing the seam, said window extending the length of said detector and exposing said detector surface to backscattered electrons, said window being of minimum width at the point closest to the impingement position and increasing in width along a secant curve at points more distinct from said impingement position.

5. An apparatus in accordance with claim 1 wherein said relative m-ovement means includes an yamplifier responsive to the seam position voltage to vary the periodic signal to deiiect the beam onto the seam.

6. A detector in accordance with claim 1 in which said detector is developed as a semicircular surface having a radius R and a center coincident with the impingement position of said beam, and in which said detector surface is developed as a Faraday cup.

7. A detector in accordance with claim 1 in which said detector is developed as a surface extending in a single plane, a shield around said detector surface and having a window exposing said detector surface to said impingement position of said beam, said window having a minimum dimension at its closest point to said impingement position and progressively widening at increasing distances therefrom along a curve equal to a constant time the distance from the origin multiplied by a secant curve.

8. A detector in accordance with claim 1 in which said detector is developed as a circular detector surface, a shield surrounding said detector surface and having a window opening in said shield along the side facing the impingement position of said beam.

9. A method for tracking and welding a seam formed between abutting workpieces to be welded along the seam by a beam of electrons comprising directing a focused beam of electrons at the workpieces and the seam with a cross-sectional shape near the seam commensurate with the width of the seam for at least partial entry thereof, collecting reliected electrons caused by the impingement of the beam onto the workpieces to generate a voltage proportional to the collected electrons,

deecting the beam across the workpieces and the seam to cause a pulse type reduction in the `magnitude of the voltage generated by the collected electrons as the beam crosses the seam,

comparing the pulsed voltage with a voltage representative of the deliection step to produce a voltage representative of a desired position of the seam relative to the beam, and

adjusting the relative position between the beam and the seam to direct the beam at the seam for welding thereof. 10. The method as recited in claim 9 wherein the beam cross-sectional shape is adjusted less than the width of the seam to provide a high signal-to-noise ratio of the pulse in the pulsed voltage.

I1. A method of tracking a seam formed between abutting workpeces comprising:

directing a beam of electrons toward the workpieces and the seam, the beam having an intensity less than that utilized for working the workpiece material and having a cross-sectional size near the workpieces substantially commensurate with the width of the seam;

collecting reflected electrons caused by the impingement of the beam onto the workpieces to generate a manifestation representative of the reflected electron current intensity;

deflecting the beam across the workpieces and the seam to cause a pulse type reduction in the manifestation generated by the collected electrons as the beam crosses the seam;

comparing the pulsed manifestation with a second manifestation representative of a beam defiection reference to produce a third manifestation representative of the position of the seam with respect to the beam dcfiection across the workpieces; and

adjusting the relative positions of the seam and the beam in response to the third manifestation to position the beam on the seam.

12. The method of claim II wherein the beam` defiection reference corresponds with the undefiected position of the directed beam.

I3. A method of tracking a weld seam between Iwo workpieces comprising:

generating and directing a beam of electrons to impinge 0n the workpieces in the vicinity of the seam to be welded and produce omnidirectional bac/scattered electrons, the beam having an intensity below that for welding the workpieees;

scanning the beam across tlze seam and the workpiece surfaces adjacent to the seam whereby the scanning beam intersects the seam;

detecting omnidirectional backscattered electrons entitled from the workpieces as the beam is scanned to generate a manifestation of the omnidirectional backscatterea electrons modulated by at least partial entry of the beam within the seam at the intersection of the scanning beam and the seam;

comparing the modulated manifestation of the backscattered electrons with a second manifestation representative of a known reference position of the scanned beam to determine the deviation of the intersection from the known reference position; and

adjusting the position of the seam and the two workpieces in response to the deviation from the known reference position to place the seam at a desired position with respect to tlze known reference position.

14. The method of claim l3 wherein the known reference position of the scanned beam corresponds to the center position of the beam scan.

I5. The method of locating a weld seam between two workpieces by means of a focussed low intensity beam of electrons comprising.'

least partial entry of the beam within the seam at the intersection of the sweep path and the seam; and correlating the occurrence of the signal modulation at the intersection of the sweep path and the seam; and a known reference position of the beam on the sweep path to determine the displacement of the seam from the known reference position whereby the seam is located with respect to the beam of electrons impinging on the workpieces. 16. A method of tracking and welding a seam between two workpieces comprising.'

generating and directing a low intensity tracking beam along a reference axis from a beam generating means to impinge on the workpieces in the vicinity of the seam as the workpieces and the tracking beam move with respect to one another;

deflecting the electron beam with respect to the axis and along a path intersecting the seam;

detecting reflected electrons emitted from the workpieces as the beam is deflected to generate a reflected electron signal modulated by at least partial entry of the beam within the seam as the beam traverses tlze intersection of the path and the seam;

referencing the modulated signal with a manifestation of the point of incidence of the reference axis on 15 the workpieces to determine the deviations of the seam from the trace of the reference axis on the workpieces; and

generating and directing a high intensity welding beam from the beam generating means to impinge on the workpieces in the vicinity of the seam and positioning the workpieces and welding beam with respect to one another in response to the deviations to provide a coincidence of the seam and the welding beam as tlte workpieces are welded along the seam by the welding beam.

References Cited The following references, cited by the Examiner, are of record in the patented file 0f this patent or the original 30 patent.

UNITED STATES PATENTS 3,146,335 8/1964 Samuelson 219-121 3,148,265 9/1964 Hansen 219-121 ,g5 3,152,238 lll/1964 Anderson Z50-49.5 3,196,246 7/1965 El-Kareh 219-121 3,291,959 12/1966 Schleich et al. 219-121 3,347,701 10/l967 Yamagishi et a1. 219--121 40 ANTHONY BARTIS, Primary Examiner U.S. Cl. X.R. 

